1. Field of the Invention
The present invention relates to braking apparatus for arresting the rotation of vehicle wheels, more particularly to braking apparatus for towed vehicles such as trailers, and most particularly to an electromechanical hydraulic trailer braking system wherein the entire system may be immersed in water without electrical failure and loss of function.
2. Discussion of the Related Art
As used herein, a "trailer" is a non-powered wheeled vehicle intended to be towed by a powered, or towing, vehicle. A trailer, therefore, is a slave to the vehicle to which it is attached, and that vehicle is responsible for both acceleration and deceleration of the trailer. For light-duty trailers, the mechanical rigidity of the trailer hitch in the direction of travel generally can provide adequate braking control of the trailer via the brakes of the towing vehicle. However, for larger, medium- or heavy-duty trailers whose loaded weight may approach or exceed the weight of the towing vehicle, it is desirable to provide a braking system for the trailer itself, which system should be responsive to signals from the towing vehicle. Even for light-duty trailers in some applications, it can be desirable to provide brakes, especially via retro fit, which are effective, relatively inexpensive, and simple to install.
Mechanically-actuated vehicle brakes are well known. Typically, a lever within the vehicle wheel assembly and connected to a foot pedal or a handle actuates a pair of pivotable brake shoes outward to sliding engagement with a rotating drum to which the vehicle wheel and tire are attached. Progressively increasing force on the lever provides progressively increasing braking action of the shoes against the drum. This basic arrangement of components was typical for mechanical brakes for most trucks and automobiles before the industry-wide conversion to hydraulic brakes around 1940. It is still used as a mechanical parking brake on some vehicles to this day.
Electric trailer brakes typically are electrically-actuated versions of mechanical brakes. In simplest form, an electromagnet is connected to the shoe-actuating lever and is movably mounted on the stationary backer plate of the wheel assembly. Energizing the electromagnet causes it to be attracted to the inner surface of the steel brake drum. Such attraction causes the lever to be moved and thus induces braking action by the shoes.
A serious shortcoming of known electrically-actuated brakes is the vulnerability of the electromagnet to wear or damage whereby the magnet can be electrically shorted, causing the brakes to become inoperative. It is known for the electromagnet to become worn from rubbing contact with the drum, and for the insulation on the electric wire turnings in the magnet to be thus abraded, allowing a short circuit to develop in the magnet. Being located in the wheel assembly, the electromagnet is exposed to road moisture and contaminants, and especially to water, as when the trailer is used to launch or retrieve a boat at an inclined launching ramp.
Another serious shortcoming is the relatively poor braking power generated by the electromagnet operating on the lever, since the size of the electromagnet is limited by the space within the brake drum. Typically, such a braking system is only marginally effective in assisting the towing vehicle in stopping a trailer. Further, it has no braking action when power to the electromagnet is turned off; thus it cannot be used as a parking brake when the trailer is unhitched from the towing vehicle.
Improved brakes wherein the wheel-mounted electromagnet is obviated have been proposed. Typically, one or more hydraulic cylinders replaces the actuating mechanism of a mechanical brake. The wheel cylinder is responsive to changes in pressure in a hydraulic feed line from a master cylinder remote from the wheel either to drive pivotable brake shoes outwards against the brake drum or to urge caliper-mounted pads into contact with a wheel-mounted disc.
In applying this technology to a trailer which typically is connectable to a hauling vehicle via a temporary ball-and-socket hitch, it is desirable that the hydraulic elements be self-contained on the trailer and that only an electric cable capable of carrying signals from the towing vehicle be connected or disconnected at the hitch. Such a trailer braking system, therefore, requires electromechanical means on board the trailer for converting electrical impulses from the towing vehicle into hydraulic action of the brakes. Various such mechanisms have been proposed.
In some trailer braking systems, slight relative motion at the hitch between the towing vehicle and the trailer provides the hydraulic pressure to apply the brakes of a towed vehicle when the towed vehicle moves forward (surges) relative to the towing vehicle when the brakes of the latter are applied, as disclosed for example in U.S. Pat. No. 3,768,606 issued Oct. 30, 1973 to Mizen et al. Since a trailer brake responsive to relative motion between the towed and towing vehicles must be capable of discriminating between real braking needs and ordinary surge in use, including backing and parking of the trailer, this system includes a complicated arrangement of springs and pivots as well as a mode-enabling solenoid.
Another system uses inertial forces on the trailer, responsive to velocity changes in the towing vehicle, to actuate a trailer's brakes. For example, U.S. Pat. No. 3,570,633 issued Mar. 16, 1971 to Garnett discloses a hydraulic brake actuator wherein a solenoid energized by the towing vehicle's brake light circuit opens a hydraulic valve in a hydraulic braking system onboard a trailer. Hydraulic pressure for braking is generated by inertial forces on the trailer acting through the hitch when the towing vehicle is braked. This system may provide little braking action at low vehicle velocities as, for example, during parking or in loading or unloading a trailered boat on a launching ramp.
It is known, for example, to provide an electric-powered hydraulic system on board the trailer. U.S. Pat. No. 4,174,869 issued Nov. 20, 1979 to Hipps discloses a trailer braking system wherein the hydraulic output of a hydraulic gear pump is passed to the wheel cylinders past a solenoid control valve whose position is set in response to the braking force applied to the brakes in the towing vehicle.
It is also known, for example, to provide a linear motor to actuate a hydraulic master cylinder in an auxiliary braking system, although not in a trailer braking system. See U.S. Pat. No. 4,077,487 issued Mar. 7, 1978 to Misinchuk.
In general, known embodiments of hydraulically-operable trailer brakes can be expensive to manufacture and costly to maintain because of the inclusion of many specially-made precision parts such as pumps, valves, switches, springs, levers, and sensors. In addition, known brakes can be uneven in braking action, and can be vulnerable to hazards such as dust, rain, and water.
Thus, there is a need for an electromechanical hydraulic trailer braking system which is simple to assemble from readily available components, is inexpensive to provide to a new or existing trailer, and is reliable under adverse conditions of use.